The present invention relates in general to well logging detectors of the scintillation crystal type, and more particularly to a ruggedized detector characterized by its high shock resistance. The detector in accordance with the invention is especially useful in measurement-while-drilling (MWD) applications wherein high shock loads on the detector are common.
U.S. Pat. Nos. 4,004,151; 4,158,773; 4,360,733; 4,383,175 and 4,764,677 all illustrate well logging detectors of the scintillation crystal type. These U.S. patents are owned by the assignee of the present invention, and are incorporated by reference herein in their entireties.
In detectors of the type disclosed in the above-noted patents, a cylindrical scintillation crystal, such as a thallium-activated alkali halide (e.g. sodium iodide) crystal, is coaxially contained within, and is hermetically sealed within, a cylindrical metal housing typically formed from stainless steel. One end of the housing has a light transparent window portion. When ionizing radiation, such as gamma radiation, impinges on the crystal,light pulses, i.e. photons, are generated within the crystal. These radiation induced light pulses exit the detector via the window portion of the detector housing. The exiting light pulses in turn are detected by an associated photomultiplier tube whose output is an electrical signal that can then be analyzed to determine the characteristics of the radiation impinging on the scintillation crystal.
In four of the above noted patents, namely U.S. Pat. No. 4,004,151; 4,360,733, 4,383,175 and 4,764,677, a compression spring applies a biasing force against one end of the crystal to maintain the other end of the crystal in optical coupling relationship with the window portion of the detector housing. The compression spring is necessary to accommodate substantial thermal expansion and contraction of the crystal within the detector housing that occurs during well logging. Typically, aluminum oxide powder, which is light reflective, is packed between the outer surface of the cylindrical crystal and the inner surface of the cylindrical housing. The packed powder serves to support and maintain the crystal at its coaxial position within the housing. It also acts as a shock absorber to protect the crystal.
Should the crystal move away from and separate from the window portion of the detector as a result of shock forces on the detector, the aluminum oxide powder could migrate between the scintillation crystal and window portion thus deleteriously affecting optical coupling therebetween. To preclude movement of the crystal away from the window portion of the detector, the aforementioned compression spring applies a large magnitude biasing force, for the most strenuous shock environment, on the order of 1,000 times the weight of the crystal, i.e. for a 1,000 g tolerant detector--1,000 times the crystal mass. For example, a one pound crystal having a two inch diameter would have applied to its nonwindow end a spring force of approximately 330 psi. High shock forces on the detector tending to move the crystal away from the window portion are thus resisted by the compressed biasing spring.
Problems arise due to the high biasing forces required to maintain the optical coupling interface between the crystal and the window portion. First, the high biasing force applied to the crystal is necessarily transferred and applied to the window portion. Thus, both the crystal and the window portion are under stress induced by the biasing spring, and can fail especially under the high-thermal transients frequently experienced during well logging. Secondly, shock induced movement of the crystal against the window portion, as opposed to away from it, can blow out the window portion and/or fracture the crystal which in effect is rammed against the window under such shock induced movement. This is because the force on the crystal and window portion are the combination of the spring force and the shock induced inertial force or g-force of the crystal against the window portion. Also, under shock induced vibrations, the packed aluminum oxide powder can shift inside of the detector so as not to properly support the crystal. The shifting powder can also grind detector components.
The present invention substantially minimizes the abovenoted problems, and provides a highly ruggedized, shock resistant detector.